At present, handheld electronic devices such as cellphones and tablet computers tend to be developed towards a higher screen proportion (proportion accounted by a screen on a front panel). For instance, taking the cellphones as an example, GOA technology has been developed in order to reduce the width of a frame at both left and right sides of a front panel of the cellphone. As to both upper and lower sides of the front panel, in order to realize some current mainstream functions, such as self-timer, automatic screen-off during talking, automatically adjusting the brightness of the cellphone screen and other functions, corresponding functional elements such as a front-facing camera, a distance sensing means, a brightness sensing means and an earphone still have to be provided on the front panel, therefore, some positions of the front panel are undoubtedly occupied. In recent years, appearance of the flexible display screen provides a space for further increasing the screen proportion.